How to measure the speed of heavy AC motors

1. Mechanical speed measurement method
Centrifugal tachometer:
Principle: The centrifugal tachometer measures the speed by using the relationship that centrifugal force is proportional to the speed. When the shaft of the tachometer is connected to the shaft of the motor through a coupling or other means, the rotor of the tachometer deflects under the action of centrifugal force, and the speed value is indicated on the dial through the pointer.
Operation method: Align the shaft of the tachometer with the shaft of the motor and connect them well to ensure that the connection is firm and not loose. Start the motor, and after the motor runs stably, read the pointer indication value on the tachometer. Note that during the measurement process, the tachometer and the shaft of the motor should be kept in the same straight line to avoid errors due to different axes.
Features: This method is simple and intuitive, with low cost, but relatively low accuracy, and needs to be directly connected to the motor shaft, which is not very convenient to operate. It is suitable for occasions where accuracy requirements are not high.
Tachometer generator:
Principle: The tachometer generator is a device that converts the speed into a voltage signal. It is coaxially connected to the motor. When the motor rotates, the tachometer generator generates a voltage signal proportional to the speed. By measuring the size of this voltage signal, the speed of the motor can be determined.
Operation method: Install the tachometer generator on the shaft of the motor and connect its output to a voltmeter or other measuring instrument. Start the motor, measure the voltage value output by the tachometer generator, and then convert the voltage value into a speed value according to the characteristic curve or calculation formula of the tachometer generator.
Features: The tachometer generator has relatively high measurement accuracy, stable output signal, and can achieve long-distance transmission. But it requires additional installation and connection, which increases the complexity of the system.
2. Electrical speed measurement method
Frequency method:
Principle: For AC motors, their speed is related to the power supply frequency and the number of pole pairs of the motor. By measuring the power supply frequency of the motor, the speed of the motor can be calculated. For example, for asynchronous motors, the speed formula is, where is the speed, is the power supply frequency, is the number of pole pairs, and is the slip rate.
Operation method: Use a frequency meter to measure the power supply frequency of the motor. If it is a three-phase AC motor, the power supply frequency of one phase can be directly measured. Then, according to the number of pole pairs and slip rate of the motor (which can be estimated by the motor nameplate or empirical value), the speed of the motor is calculated.
Features: This method does not require direct contact with the motor, is convenient to measure, and has high accuracy. However, it is necessary to accurately know the number of pole pairs and slip rate of the motor, and the measurement results may be affected by unstable power supply frequency.
Pulse counting method:
Principle: Install a photoelectric encoder or Hall sensor or other device on the shaft of the motor. When the motor rotates, these devices will generate a pulse signal proportional to the speed. By counting the number of pulses within a certain period of time, the speed of the motor can be calculated.
Operation method: Install the photoelectric encoder or Hall sensor on the shaft of the motor and connect its output to a device such as a counter or microprocessor. Start the motor, record the number of pulses within a certain period of time, and then calculate the speed of the motor according to the resolution and time interval of the encoder or sensor.
Features: The pulse counting method has high measurement accuracy, fast response speed, and can realize real-time monitoring. However, it is necessary to install special sensors and counting equipment, and the cost is relatively high.
In practical applications, the appropriate speed measurement method can be selected according to the specific situation. At the same time, in order to improve the measurement accuracy, multiple methods can also be used for comparative measurement, and the measurement results can be calibrated and corrected.
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